The present invention relates to a process for the preparation of a porous ceramic body having very fine cells having an average pore size of about 0.08 .mu.m (hereinafter referred to as the "micro-cell"). More particularly, the present invention relates to a process in which a porous ceramic body having a high porosity and a fine pore size, which is homogeneous and is neither deformed nor cracked, can be prepared with a high productivity by a simple operation.
Recently, demands for porous ceramic bodies having the micro-cells as the material of filters, electrodes and electrolyte holders of fuel cells, catalyst carriers and the like have been increasing rapidly.
These porous ceramic bodies are generally prepared by molding a mixture of a ceramic powder with a combustible substance and firing the molded body.
According to this process, a porous body having an average pore size of about 2 to about 3 .mu.m and a porosity of about 80% can be obtained. However the upper limit of the porosity of a porous body having the micro-cells is about 65%, and no micro-cellular porous ceramic body having a porosity exceeding this level can be prepared by this process.
This is because although a fine organic powder should be added for formation of the micro-cells, the molded body inevitably collapses in the firing step, if the amount of the fine organic powder exceeds a certain limit.
According to this process using an organic powder, since it is difficult to reduce the particle size of the powder to several .mu.m or below, micro-cells having a diameter smaller than several .mu.m are hardly formed. Moreover, this process is defective in that since the organic powder is elastic, the molded body is liable to be expanded and cracked, when the molding pressure is released after the molding operation.
As another process for preparing a porous ceramic body, there is known a process in which a polyurethane foam is used as the base material, from which the walls of the cells are removed and the foam is then coated with a ceramic material to give a porous body having open cells. The porosity of the porous body prepared according to this process is 85 to 90%, but the pore size is as large as about 1 to about 4 mm so that no micro-cellular porous body can be prepared.
Furthermore, as the process for the preparation of a porous body which is dense and has the micro-cells, there is known a process in which up to 10% by weight of water or an aqueous solution of a binder is added to a starting ceramic powder, the mixture is press-molded and the molded body is fired.
According to this process, a sintered body having the micro-cells can be obtained and when the amount of water is increased to 18 to 22% by weight, extrusion molding becomes possible but the porosity of the green molded body is 40 to 50% and no porous body having a porosity higher than 50% can be obtained. Moreover, when the amount of water is increased to 22 to 30% by weight, the starting material/water mixture forms a slurry and slip casting is possible, but the porosity of the green molded body is still of the order of about 40 to about 50% so that no molded body having a porosity higher than 50% can be obtained.
As is apparent from the foregoing description, and porous ceramic body having a high porosity, a fine pore size and an excellent quality cannot be obtained according to any of the conventional processes.